The mechanism by which Ca2 ion is transported across the cardiac cell membrane is investigated. AK ion-linked Ca2 ion transport system was postulated, which is capable of transporting Ca2 ion against its electrochemical gradient. Evidence was obtained from comparing the K ion efflux and accumulation (using K ion-selective microelectrodes) with the magnitude of the tension development. In the presence of of Ni2 ion (a Ca2 ion antagonist) the effectiveness of Ca2 ion transport system was examined. It was found that Ni2 ion depresses tension and renders the membrane highly Na ion selective. Under such conditions TTX supresses tension and the time dependent outward current. The decrease in magnitude of time dependent current was linearly related to developed tension. The decrease in outward current also was quantitatively related to the decrease in K ion efflux. These experiments provide critical evidence for the existence of a K ion related Ca2 ion transport system. BIBLIOGRAPHIC REFERENCES: Goldman, Y. and M. Morad. Regenerative repolarization of the frog ventricular action potential: A time and voltage dependent phenomenon. J. Physiol. 268:1-37, 1977. Goldman, Y. and M. Morad. Measurement of transmembrane potential and current in cardiac muscle: A new voltage clamp method. J. Physiol. 268:38-80, 1977.